In a car with a carbureted engine, the vacuum in the intake manifold provides additional force. Since the described diesel models do not provide vacuum in this place, they have a special vacuum pump for this purpose (see fig.). The pump is connected to the brake booster by an air hose. When braking, a special valve is activated and the force from the pressure difference is added to the force of the foot on the pedals (atmosphere and vacuum in the intake manifold) on the amplifier membrane.
Vacuum pump (2) creates a relative vacuum necessary for the operation of the brake booster. To do this, the pump through a hose with a check valve (1) sucks air from the booster and directs it through a second hose (3) to the engine crankcase. This is done to prevent noise in the pump. The newer pumps do not have this hose.
If the engine is not running, the booster is also unable to provide additional braking force. Therefore, it is necessary to press the pedal harder if, for example, the car is towed. With the engine off, you still have some margin for a few short brakings thanks to the available vacuum; only then do you fully tighten the calf muscles.
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